Ecology and diversity of freshwater shrimps in Banco National Park, Côte d'Ivoire (Banco River Basin)
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Knowledge and Management of Aquatic Ecosystems (2009) 393, 05 http://www.kmae-journal.org
c ONEMA, 2010
DOI: 10.1051/kmae/2009020
Ecology and diversity of freshwater shrimps in Banco
National Park, Côte d’Ivoire (Banco River Basin)
I.A. Camara(1) , M.K. Konan(1) , D. Diomandé(1) , E.O. Edia(1) , G. Gourène(1)
Received October 16, 2009 / Reçu le 16 octobre 2009
Revised November 27, 2009 / Révisé le 27 novembre 2009
Accepted November 30, 2009 / Accepté le 30 novembre 2009
ABSTRACT
Key-words: Shrimps are an important component of river invertebrate assemblages in
ecology, tropical freshwater where they are a controlling group in food webs. In or-
diversity, der to determine shrimp diversity in the Banco Basin (Côte d’Ivoire, West
freshwater Africa) and examine whether the patterns of species distribution were re-
shrimps, lated to environmental conditions, six sites were monitored. Sampling was
Banco River conducted in 2008 during February, May, September and November using
Basin, a long-handled net (25-cm diameter, 2-mm mesh). For each site, we mea-
Banco National sured environmental variables (conductivity, pH, water temperature, dis-
Park, solved oxygen, turbidity, current speed, and water depth and width). Three
Côte d’Ivoire shrimp species, Macrobrachium thysi (Powell 1980), Macrobrachium dux
(Lenz 1910) and Desmocaris trispinosa (Aurivillius 1898) were identified.
D. trispinosa was the largest distributed (more than 50% of occurrence)
and the most abundant (67% of total number of specimens caught). It
was followed by M. thysi with 47% and 32% of occurrence and abun-
dance, respectively. Canonical correspondence analysis (CCA) showed a
spatial distribution of the shrimp community, significantly influenced by
width, depth, conductivity, type of substrate, turbidity and dissolved oxy-
gen. Due to the fact that the Banco River hosts an endemic species in
Côte d’Ivoire (Macrobrachium thysi), the conservation of integrity of this
basin was recommended.
RÉSUMÉ
Écologie et diversité des crevettes d’eau douce dans le Parc National du Banco
(Bassin du Banco, Côte d’Ivoire)
Mots-clés : Les crevettes ont un rôle très important dans la chaîne trophique des eaux douces.
écologie, La diversité des crevettes et leurs relations avec les paramètres environnementaux
diversité, ont été étudiées dans le bassin du Banco (Côte d’Ivoire, Afrique de l’Ouest). Les
crevettes d’eau crevettes ont été échantillonnées mensuellement pendant quatre mois en 2008
douce, (février, mai, septembre et novembre) à l’aide d’une épuisette (25 cm de diamètre,
bassin de la 2 mm de maille). À chaque échantillonnage, des paramètres environnementaux
ont été évalués. Trois espèces de crevettes ont été capturées : Macrobrachium
rivière Banco,
thysi (Powell 1980), M. dux (Lenz 1910) et Desmocaris trispinosa (Aurivillius 1898).
Parc National du
L’espèce D. trispinosa est la plus largement distribuée (plus de 50 % d’occur-
Banco,
rence) et la plus abondante (67 % d’abondance relative). Elle est suivie de M. thysi
Côte d’Ivoire avec une occurrence et une abondance respectives de 47 % et 32 %. L’analyse
(1) Laboratoire d’Environnement et de Biologie Aquatique, UFR-Sciences et Gestion de l’Environnement,
Université d’Abobo-Adjamé, 02 BP 801 Abidjan 02, Côte d’Ivoire, camadams80@yahoo.fr
Article published by EDP SciencesI.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
canonique des correspondances montre que la distribution spatiale des crevettes
dans le bassin du Banco est fortement influencée par la largeur, la profondeur, la
conductivité, le type de substrat, la turbidité et l’oxygène dissous. Compte tenu
du fait que la rivière Banco héberge une espèce endémique à la Côte d’Ivoire
(Macrobrachium thysi), la conservation de l’intégrité de ce bassin est fortement
recommandée.
INTRODUCTION
Shrimps are critical species in stream food webs because they serve as intermediate con-
sumers, linking production of periphyton and detritus with higher trophic groups (Browder
et al., 1994; Frederick and Spalding, 1994). Yet the shrimps represent an important food re-
source for carnivorous fishes (Resende et al., 1996). The Banco National Park (BNP) is located
in the middle of Abidjan, the economic capital of Côte d’Ivoire. This park, founded in 1953,
is a tropical rain forest with primary relicts of evergreen forest (Hall and Swaine, 1981; Parren
and De Graaf, 1995). The Banco River situated in this park is subjected to anthropogenic
disturbance due to sewage draining by canals coming from the neighbouring cities of BNP.
This disturbance represents a menace for the river integrity and the aquatic communities. De-
spite the conservation role of BNP, few studies (Daget et Iltis, 1965; Powell, 1980) have been
devoted to aquatic fauna. Concerning shrimps, the only literature concerns the description
of Macrobrachium thysi by Powell (1980). To our knowledge, no study, so far, has dealt with
diversity and ecological requirements of shrimp of this river.
The objectives of this study are (i) to present a comparative account of the physicochem-
ical parameters, and the density and wet biomass of the shrimps between upstream and
downstream stations of the Banco River channel and the permanent pool stations, and (ii) to
investigate the relationships between shrimp density and environmental parameters of the
Banco River Basin.
MATERIALS AND METHODS
> STUDY AREA AND SAMPLING STATIONS
The Banco River Basin (approximately 9 km) is located between 5◦ 21’–5◦ 25’ N and
4◦ 01’–4◦ 05’ W in the BNP (3000 ha) (Figure 1). The climate of the BNP is typical of equa-
torial rain forest, comprising four seasons (Girard et al., 1971): a long dry season (December
to March), a long rainy season (April to July), a low dry season (August to September) and a
low rainy season (October to November).
Sampling was conducted in 2008 during February, May, September and November. Six sam-
pling stations were selected and each was sampled once in each sampled month. Four sites
(S1, S2, S3 and S5) were situated along the main river channel, while the other two (S4 and S6)
were permanent pools. Stations S1 and S2 were located in the upstream and were commonly
characterised by clean water, and the presence of fragmented leaves, woody debris, riparian
vegetation (Turraenthus africanus, Petersianthus macrocarpus and Dacryodes klaineana) and
macrophyte flora (Thaumatococcus daniellii), with a sandy-silty substratum. In the midstream
station (S3), the water was cloudy, smelly, and likely to contain a lot of suspended matter due
to arrival of waste water. The substratum in S3 was mainly sand and lacking in riparian vegeta-
tion. Musanga cecropioides and Xanthosoma sp. were the marginal vegetations found at this
station. At the downstream station (S5), banks were well defined with marginal grassy vege-
tation (Cyclosorus striatus and Nephrolepis biserrata). The river bed was rocky to sandy with
cloudy water. Water was clear (S4) to dark (S6) in the permanent pool stations. These sites
were defined by an abundance of roots and large quantities of plant detritus with a sandy to
silty bed and high vegetation coverage. The station S6 was covered by Indian bamboo trees
(Bambusa sp.).
05p2I.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
Figure 1
Map of the Banco National Park showing the study stations.
Figure 1
Carte du Parc national Banco situant les stations d’étude.
> ENVIRONMENTAL VARIABLES
Thirteen environmental variables were used to describe habitat conditions at each sampling
site. Measurements of all variables were made in situ between 7 h and 12 h before shrimp
sampling. Conductivity (µS·cm−1 ), water temperature (◦ C), dissolved oxygen (mg·L−1 ) and pH
were measured using a multiparameter apparatus (WTW 840i). Turbidity (NTU) was measured
using a turbidimeter (AQUA LYTIC CD 24). The habitat variables included current veloc-
ity (m·S−1 ), which was taken in mid-channel on five occasions by timing a floating object
(polystyrene cube) over a five-metre stretch of the river. The value of current velocity was the
average of the five trials. Water depth (m) and width (m) were measured (average of five trials)
to the nearest centimetre using a decametre. Canopy cover (%), substrate type (mud, sand,
gravel), and woody debris (%) were estimated visually at each sampling site as described by
Gordon et al. (1994), and Rios and Bailey (2006).
05p3I.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
> SHRIMP COLLECTION AND IDENTIFICATION
Shrimps were collected using a long-handled net (25-cm diameter, 2-mm mesh). Four sam-
ples were obtained at each site during the sampling period. The material retained in the net
was transferred to a white tray, and all shrimp individuals were sorted, placed in plastic bot-
tles and preserved with 70% ethanol. In the laboratory, the shrimps were sieved by using a
0.5-mm sieve sorted into a Petri dish. All specimens caught were identified under a stereomi-
croscope according to Monod (1966, 1980) and Powell (1980), counted, and weighed.
> DATA ANALYSES
The number of individuals per species obtained in the sample depends on the space of time
of the sampling. We standardised all data by conversion to densities. Shrimp density was
defined as the number of individuals per 30 min. The frequency of occurrence (FO) was used
to classify species according to Dajoz (2000).
Before performing the comparison test, the normality of data was checked by the
Kolmogorov-Smirnov test (p > 0.05 at all stations). The Mann-Whitney U-test was performed
to compare densities between sampling sites using the STATISTICA 7.1 computer package.
A significance level of p < 0.05 was considered.
Correlations between environmental variables and taxonomic richness were tested by the
Spearman correlation test. In order to study relationships between environmental variables
and the structure and dynamics of the shrimp community, Canonical Correspondence Analy-
sis (CCA) was performed based on the data matrix of shrimp densities. CCA is an ordination
method effective in directly revealing correlations between the spatial structure of commu-
nities, and environmental factors that might be responsible for that structure (Ter Braak and
Smilauer, 2002). Environmental variables and shrimp data were log10 (x + 1) transformed prior
to analysis. Monte Carlo permutations (500) were done so as to identify a subset of measured
environmental variables, which exerted significant and independent influences on shrimp dis-
tribution at p < 0.05. CCA was performed using CANOCO 4.5 (Ter Braak and Smilauer, 2002).
RESULTS
> ENVIRONMENTAL VARIABLES
The mean values of environmental variables are shown in Table I. The conductivity varied from
22 µS·cm−1 (S1) to 61.5 µS·cm−1 (S4). The water of Banco River was acid with low variation in
pH (4.5 (S1)–6.1 (S3)) between sampling sites. It was the same for water temperature, which
ranged between 25.87 ◦ C and 26.43 ◦ C. The lowest dissolved oxygen values were recorded
in permanent pools (≤ 2.64 mg·L−1 ) and the highest values were observed in river channel
stations (≥ 4.12 mg·L−1 ). Generally, low turbidity (≤ 13.15 NTU) was observed in upstream
areas, while high values (≥ 30 NTU) were recorded in the other sites. The upstream stations
(S1 and S2) presented low values of velocity (≤ 0.25 m·S−1 ), while high values (≥ 0.33 m·S−1 )
were recorded in the middle and downstream stations (S3 and S5). In the permanent pools,
width was almost constant, while it increased from upstream to downstream (2.77 to 4.61 m)
in the river channel. The depth in the study areas varied from 0.14 m (S1) to 0.48 m (S5).
> COMPOSITION AND DISTRIBUTION OF SHRIMP SPECIES
During this study, three shrimp species, Macrobrachium thysi, M. dux (Palaemonidae) and
Desmocaris trispinosa (Desmocarididae), were collected. M. thysi and D. trispinosa were the
species recorded in the upstream sites (S1 and S2). No shrimp was caught in the midstream
station (S3). In the downstream station (S5), the three species were represented in samples,
05p4I.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
Table I
Environmental parameters evaluated in the study area. Cond.: conductivity, Temp.: temperature, DO:
dissolved oxygen, Turb.: turbidity, S.D.: standard deviation, (-): not measured.
Tableau I
Paramètres environnementaux mesurés dans les sites d’étude.
Stations Cond. pH Temp. DO Turb. Velocity (m·S−1 ) Width (m) Depth (m) Slope (◦ )
Mean 22.0 5.405 25.875 4.6 13.15 0.21 2.77 0.14
S1 Ranges 20–24 4.5–6.05 25.3–26.7 4.02–5.49 11.9–14.3 0.20–0.23 2.5–2.87 0.13–0.15 45–90
S.D. 1.6 0.7 0.6 0.7 1.0 0.0 0.2 0.8
Mean 26.0 5.3575 25.875 5.02 9.825 0.25 3.11 0.23
S2 Ranges 22–36 4.73–5.75 25–26.5 4.59–5.38 7.1–12.9 0.22–0.3 2.9–3.53 0.20–0.26 45–90
S.D. 6.7 0.4 0.6 0.4 2.4 0.0 0.3 2.4
Mean 45.0 5.62 26.43 4.12 43.9 0.45 3.66 0.28
S3 Ranges 31–71 5.25–5.97 26.3–26.6 4.06–4.22 17.7–57 0.38–0.54 3.25–3.94 0.24–0.32 5–45
S.D. 22.5 0.4 0.2 0.1 22.7 0.1 0.4 4.1
Mean 61.5 5.555 26.325 2.635 45.13 – – –
S4 Ranges 48–81 4.96–5.9 26.1–26.5 1.4–3.8 14.4–93 – – – 0.05). For the other sampling ar-
eas, the highest density values were recorded for D. trispinosa (S4: 261 ind·30 min−1 , S5:
183 ind·30 min−1 , S6: 60 ind·30 min−1 ) (Table II). The Mann-Whitney U-test revealed signifi-
cant difference (at least p < 0.05) in D. trispinosa densities between sampling stations except
for the upstream stations (S1 and S2).
> RELATIONSHIPS BETWEEN ENVIRONMENTAL VARIABLES
AND SHRIMP COMMUNITIES
The Spearman correlation analysis indicated that Macrobrachium thysi was significantly in-
fluenced by conductivity, dissolved oxygen, turbidity, depth and mud (Table III). Width, depth
and current velocity had a significant correlation with M. dux. On the other hand, D. trispinosa
was mainly influenced by current velocity, sand and gravel.
The results of redundancy analysis revealed that the relationships between the three shrimp
species and their habitat conditions follow mainly the first two axes (Figure 2). These two
axes accounted for 95.9% of the total variance and evidenced a spatial pattern in the shrimp
assemblage of Banco River Basin. The midstream station (S3) where no shrimp was caught
was excluded from analysis. The first axis (F1) opposed upstream stations in positive coor-
dinates to permanent pools and downstream sites in negative coordinates. M. thysi and the
other two species (M. dux and D. Trispinosa) were also opposed according to the same in-
terpretations. High densities of M. thysi were mainly influenced by a good oxygenation level,
05p5I.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
Table II
Density (individuals per 30 min), distribution of shrimp species in Banco River Basin study stations and
results of the Mann-Whitney test. Med: median, Min: minimum, Max: maximum, (-): absence. Medians
in the same column followed by different letters are significantly different (p < 0.05).
Tableau II
Densité (individus par 30 min), distribution des espèces de crevettes dans les sites d’étude du bassin
de la rivière Banco et résultats des tests de Mann-Whitney.
Shrimp species
Stations Desmocaris trispinosa Macrobrachium thysi Macrobrachium dux
Med 15a 63.75a –
S1 Min 4.5 34.5 –
Max 45 106.5 –
Med 2.25a 69a –
S2 Min 1.5 21 –
Max 9 85.5 –
Med – – –
S3 Min – – –
Max – – –
Med 208.5b – –
S4 Min 120 – –
Max 261 – –
Med 122.25c 0.5b 3.75
S5 Min 66 0 0
Max 183 3 21
Med 34d – –
S6 Min 10 – –
Max 60 – –
Table III
Spearman correlation of shrimp densities and physicochemical data of Banco River Basin.
Tableau III
Corrélations de Spearman entre les densités de crevettes et les paramètres physico-chimiques dans le
bassin de la rivière Banco.
Shrimp species
Macrobrachium thysi Macrobrachium dux Desmocaris trispinosa
Environmental variables p r p r p r
∗∗
Conductivity –0.77 ns 0.01 ns 0.37
pH ns –0.22 ns 0.02 ns 0.37
Temperature ns –0.26 ns –0.07 ns –0.07
∗
Dissolved oxygen 0.44 ns 0.38 ns –0.35
∗∗
Turbidity –0.76 ns 0.12 ns 0.21
∗ ∗∗
Current velocity ns 0.04 0.41 –0.54
∗∗
Width ns –0.3 0.65 ns –0.19
∗∗ ∗
Depth –0.75 0.47 ns 0.38
∗∗
Mud 0.74 ns 0.27 ns –0.37
∗∗
Sand ns 0.33 ns 0 –0.64
∗∗
Gravel ns 0.03 ns 0.39 0.61
Woody debris ns 0.34 ns –0.39 ns 0.06
Canopy ns 0.12 ns –0.39 ns 0.22
r: Spearman correlation coefficient.
∗
or ∗∗ : indicates significant difference (p ≤ 0.05 or 0.01).
ns: non-significant relationship.
05p6I.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
Figure 2
Canonical correspondence analysis triplot showing species and sampling stations in relation to environ-
mental variables in Banco River Basin: Macrothysi = Macrobrachium thysi; Macrodux = Macrobrachium
dux; Desmo = Desmocaris trispinosa; station (S); number of station (1–6); sampling period in indices
(1 = May, 2 = September, 3 = October, 4 = February).
Figure 2
Analyse canonique des correspondances mettant en relation les espèces et les sites d’étude avec les
variables environnementales dans le bassin de la rivière Banco. Macrothysi = Macrobrachium thysi ;
Macrodux = Macrobrachium dux ; Desmo = Desmocaris trispinosa ; station (S) : numéro des stations
(1–6) ; période d’échantillonnage (1 = mai, 2 = septembre, 3 = octobre, 4 = février).
and high percentage of mud, sand and woody debris, while M. dux and D. trispinosa were
more associated with high values of water depth, conductivity and turbidity. The second axis
(F2) opposed the samples of downstream stations in positive coordinates to the samples
from the permanent pools in negative coordinates. This axis also opposed M. dux, which was
influenced by high values of water width and current velocity, to D. Trispinosa.
DISCUSSION
This survey revealed that the number of shrimp species (3) recorded in the Banco River Basin
is low compared with the diversity of other Ivorian rivers. Indeed, 7, 9 and 10 species were
captured in the Bia (Gooré Bi, 1998), Boubo (N’Zi et al., 2008) and Mé rivers (N’Zi et al.,
05p7I.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
2003), respectively. This difference in species richness was probably due to the rivers’ sizes.
In fact, Banco River is very small (9 km length, 4.5 m width) compared with Bia River (290 km,
150 m), Boubo River (130 km, 13.54 m) and Mé River (140 km, 13.48 m) (N’Zi et al., 2003,
2008). Moreover, a previous study showed that from 1955 to 1998, Banco Bay lost more than
20% of its water surface to silting (Anoh, 2002). This comes from streaming waters that drain
sometimes significant quantities of sand towards the bay, from free-bottomed gutters still
numerous in the urban landscape of Abidjan. These sediments, which form a deposit in is-
sues of the Banco River channel, create arrows of sand on which homeless populations settle
down (Anoh, 2002). This deposit of sand constituted a barrier for the amphidromous shrimps
(Macrabrachium vollenhovenii and M. macrobrachion) whose life cycles require upstream mi-
gration (Pyron and Covich, 2003; Benbow et al., 2004). The importance of species migration
level as a determinant of species diversity and distribution is largely documented for tropical
rivers (Hunte, 1977; Ville, 1983).
Macrobrachium thysi, an endemic species to Côte d’Ivoire (Powell, 1980), was commonly
encountered in the upstream stations and absent in the middle stream and permanent pool
stations. No shrimp specimen was collected in the middle stream station, while Desmocaris
trispinosa was sampled in all the other sampling sites. It was the only one species registered
in the permanent pool stations. Macrobrachium dux was found only in the downstream site.
This species distribution was related to the variation in habitat conditions in sampling sites.
Several studies showed that the assemblages and distribution of the benthic macroinverte-
brates, and their species richness, were usually extremely dependent on habitat heterogeneity
(Fiévet et al., 2001; Able et al., 2002; Fossati et al., 2002; Arimoro et al., 2007, Bücker et al.,
2008).
In the upstream sites, the shrimp assemblage of Banco River was dominated in number by
M. thysi. The high density of M. thysi in upstream stations was due to a good oxygenation
level and the presence of mud, woody debris and fragmented leaves. This microhabitat might
offer favourable life conditions (substratum, food) for this shrimp. This observation corrobo-
rated the results of March et al. (2001), and Usio and Townsend (2001), which showed that
crayfishes and shrimps are of fundamental importance to decomposition of plant material in
streams. In addition, the high density of this species was observed in the sites presenting a
low current velocity. This suggested that M. thysi has no ability to resist strong currents, as
shown by Fossati et al. (2002) for Caridina waberi (Atyidae). D. trispinosa was more abundant
in the downstream and permanent pool stations. This species seems to tolerate unfavourable
conditions such as low dissolved oxygen and significant organic matter, as suggested by
Cumberlidge (2005). It is well known that pools situated at a small geographical distance from
one another tend to have similar macroinvertebrate assemblages (Briers and Biggs, 2005).
Macrobrachium dux was found only in the downstream site (S5). Its distribution was associ-
ated with high values of water depth and width, and current velocity. Due to its great size and
strong pereiopods, M. dux is probably able to resist strong currents. This observation was in
agreement with the results of Fossati et al. (2002) in Atyoida pilipes from Nuku-Hiva Island
rivers.
Comparing M. thysi with D. trispinosa, our study revealed that the first species prefers clean
water with good oxygen levels, while the second seems to tolerate low dissolved oxygen
concentration. According to Rosenberg and Resh (1993), shrimps have different levels of
sensitivity to pollution and many abiotic factors in the river ecosystem. Moreover, Declerck
et al. (2005) reported that different taxonomic groups reacted differently in response to the
same chemical parameters.
The poorer water quality downstream than upstream could be attributable to several man-
induced activities such as urban runoff to surface river water due to direct or unregulated
discharges in the Banco River of sewage water from neighbouring cities.
The absence of the shrimps at the middle stream station (S3) was related to regular point
sources of pollution discharge due to the arrival of non-treated domestic sewage bringing
about the water quality deterioration. Equally, the strong currents at this station explicated
05p8I.A. Camara et al.: Knowl. Managt. Aquatic Ecosyst. (2009) 393, 05
the absence of shrimp which cannot resist such currents. In other respects, the modification
of the sandbank of the river bed by runoff at this station might justify the absence of shrimp.
Our results indicated that the shrimp diversity was low (three species). The pattern distribution
of these shrimps differs on a microhabitat scale. M. thysi required high oxygen and low current
velocity, D. trispinosa tolerated a great variation in environmental conditions, while no factor
influenced the distribution of M. dux. The middle stream areas of the Banco River, which
received wastewater from some densely populated areas and industries nearby, sheltered no
species. Thus, it is important that the regulatory authorities should implement and enforce an
appropriate strategy to monitor, regulate and protect this area of the river and its community.
ACKNOWLEDGEMENTS
We are especially grateful to the “Office Ivoirienne des Parcs et Réserves” and the “Direction
des Eaux et Forêts de Côte d’Ivoire” for the access permit to Banco National Park.
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